Performance Verification Statement for the Idronaut 305 Plus CTD pH Sensor

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Part.IVA IT-13292880153
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Our Ref.: FG/LM date 3 rd March 2015
Ref.: Performance Verification Statement for the Idronaut OCEAN SEVEN 305Plus CTD
First of all, we wish to congratulate you on the very professional, sophisticated and high-level scientific
work carried out on practical field measurements and data verification.
Foreword
1) Had we been aware of the extended time period for these tests, we could have provided replacement
sensors and more in depth training, particularly in regard to the additional control measures necessary
to ensure good quality data over repeated long deployments. These control measures would have
determined whether the reference sensor was exhausted or continuing to functioning correctly prior to
calibration and deployment.
2) We are sorry and saddened for all the sea data lost due to the battery’s failure.
Various considerations on the seawater measurements
For the last few decades, Idronaut has been selling (directly or indirectly) their pH glass sensors with the
associated reference sensors to most of the worldwide CTD multiparameter probe manufacturers.
Despite the great attention given to the CO2 exchange between the ocean and the atmosphere, it seems
that the number of scientists who make pH measurements in the field (until now, mainly with the
traditional potentiometric technique of pH glass sensors, mainly by using our pH sensors) is not
increasing, as our sales of these sensors have been almost the same for the last ten years. As we produce
about 200 pairs of pH and reference sensors per year, where probably 50% of them are dedicated to
maintaining existing instruments, it seems to us that the population of scientists that makes pH
measurements in the ocean is rather limited.
Idronaut sensors are designed for operation in sea water to depths of 7000 m or even more, if required.
This is a completely different field to that of fresh water where competition is very high and, as a
consequence, prices and robustness of sensors at depth are rather low.
Analysis of the “bad” Idronaut data
Electrochemical sensors have a finite life and for the reference sensor this is determined by the type of
sample and length of exposure. Submersible reference sensors have a limited reservoir of electrolyte unlike
good quality laboratory sensors, where the internal electrolyte can be topped up or replaced.
Idronaut KCl reference sensors have a lifetime of about 3 to 6 months when continuously immersed in sea
water. During immersion, the internal electrolyte will be gradually lost over time and possibly replaced
with ions from the sample. As time progresses, the outflow of electrolyte needed to maintain a stable liquid
junction decreases and the sensor looses its ability to maintain a stable liquid junction potential as the
temperature and ion strength of the sample (or calibration buffer) changes. In samples having a lower ion
strength, the depletion process could be accelerated.
Standard laboratory calibration and tests using NBS buffers would not have highlighted a reference sensor
where its reservoir of potassium chloride was approaching depletion.